1. Source
2. Path
3. Receiver

Without a source there obviously is no sound. Without a path, the medium through which sound passes to the receiver, there is no sound. Without a receiver, someone who hears the sound, there is no sound problem.

SOURCE

Sound is the result of rapid fluctuations of pressure, which reach a receiver. The frequency of sound is the number of times in a period of one second that the pressure changes from zero to maximum to minimum to zero, thus completing a cycle. In music it is perceived as the pitch or tone of a note. Frequencies produce sound waves; the length of the sound wave depends on the specific frequency. Humans tend to be more sensitive to high and mid range frequencies such as sirens, whistles and traffic noise. Lower frequencies tend to be less irritating.

Amplitude refers to the loudness of sound. The loudness of sound is often expressed in decibels (dB). Human hearing is impacted by the way it perceives sound levels. Higher and lower frequencies of the same magnitude can be perceived as less intense; therefore, to approximate the response of the human ear adjustments are made to account for human sensitivity to certain frequencies. These adjustments are identified as dBA's.

For the majority of people, the threshold of hearing is higher than 0 dBA, probably closer to 10 dBA. A change of 1 dBA in sound is the smallest change most people can recognize when comparing two sounds.

Many State Department of Transportation agencies claim a change of 3 dBA is the smallest change most people can recognize. A change of 10 dBA is generally thought to be "twice as loud."

Decibels are logarithmic units; therefore, multiple dB cannot be added by ordinary arithmetic means. For example, if one automobile generates 70 dB when it passes an individual, two cars passing simultaneously would not produce 140 dB. In fact, they would combine to produce 73 dB. The chart below can be used to reasonably estimate the impact of two or more noise sources together.

PATH

Although frequency and amplitude originate at the source, both are significantly altered by the physical variables in the path to the receivers. For example, walls, structures, ground absorption, atmospheric conditions such as temperature, humidity, wind and rain all contribute to changes in source noise levels before it reaches the receiver. A detailed study of the path is a critical step in understanding how to reduce noise levels at specific locations.

RECIEVER

Ultimately, we are concerned with the effect and perception of sound on the receiver. Two elements determine the sound levels upon the receiver: the sound power levels of the source and the characteristics of the path between the source and receiver. A third and critical element is the individual sensitivity of the human receiver. The individual's sensitivity plays a significant role in his perception of noise levels.

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